GENERAL LAWS OF OEIGIN AND MIGRATION. 247 



of the Upper Cretaceous had again given place to the 

 wai-m plains and land-locked brackish seas or fresh-water 

 lakes of the Laramie period (Eocene). Thus the true 

 Upper Cretaceous marks a cool period intervening be- 

 tween the so-called Upper Cretaceous (really Middle Cre- 

 taceous) and the so-called Miocene (really Lower Eocene) 

 floras of Greenland. 



This latter established itself in Greenland, and prob- 

 ably all around the Arctic circle, in the warm period of 

 the earliest Eocene, and, as the climate of the northern 

 hemisphere became gradually reduced from that time till 

 the end of the Pliocene, it marched on over both conti- 

 nents to the southward, chased behind by the modern 

 arctic flora, and eventually by the frost and snow of the 

 Glacial age. This history may admit of correction in de- 

 tails ; but, so far as present knowledge extends, it is in 

 the main not far from the truth. 



Perhaps the first great question which it raises is that 

 as to the causes of the alternations of warm and cold cli- 

 mates in the north, apparently demanded by the vicissi- 

 tudes of the vegetable kingdom. Here we may set aside 

 the idea that in former times plants were suited to endure 

 greater cold than at present. It is true that some of the 

 fossil Greenland plants are of unknown genera, and many 

 are species new to us ; but we are on the whole safe in 

 afiirming that they must have required conditions similar 

 to those necessary to their modern representatives, except 

 within such limits as we now find to hold in similar cases 

 among existing plants. Still we know that at the present 

 time many species found in the equable climate of Eng- 

 land will not live in Canada, though species to all appear- 

 ance similar in structure are native here. There is also 

 some reason to suppose that species when new may have 

 greater hardiness and adaptability than when in old age 

 and verging toward extinction. In any case these facts 

 can account for but a small part of the phenomena, which 



